99 Diseases of Pressure Equipment: Sigma Phase Embrittlement

Another form of metallurgical degradation at higher temperatures is called sigma phase embrittlement. As the name implies, a metallurgical phase change occurs in some stainless steels when they are heated above about 1000F (540C). That phase change results in a dramatic loss of toughness, and therefore, can lead to brittle fracture of the “sigmatized” equipment. The more common brittle fractures occur in cast and welded stainless steel equipment containing higher ferrite contents that have been sigmatized in service or during PWHT. The fact that it can and does occur during PWHT attests to the fact that it can occur rapidly with only a short-time in the susceptible temperature range. Typically brittle fractures of affected material occur when stresses are applied as the equipment cools to lower temperatures. For example, a cast stainless steel furnace tube support may fracture during furnace cool down; cast stainless steels can fracture like shrapnel if the tubes are shocked in service; cast stainless steel valves in high temperature service; and welded stainless steel cyclones in FCCU regenerators may end up with weld cracks as the equipment cools during shutdowns. Weld overlays of heavy wall Cr-Mo equipment have been found sigmatized and cracked after PWHT. All these examples are for austenitic stainless steels (with higher ferrite content), but ferritic and martensitic stainless steels are also susceptible.

Once again, there’s no reasonable way to detect sigmatized stainless steels other than doing impact testing or metallographic examination on failed equipment to verify what caused the brittle cracking. The key to prevention of brittle cracking from sigma phase is to select alloys that have less susceptibility to being sigmatized above 1000F, and with austenitic stainless steels that means careful ferrite control.

Have you identified stainless steel equipment in your process equipment operating at elevated temperatures that might be susceptible to sigma phase embrittlement and planned accordingly in the event that brittle failure occurs?